Traveling wave tube with planar equiangular spiral slow wave circuit

ABSTRACT

This invention concerns a more compact, lower-cost, lowervoltage, broader-band, traveling wave amplifier that has a cylindrical electron gun between closely spaced, parallel, flat surfaces of a pair of ceramic disks and a ring collector joined to the disks concentric with the electron gun. At least one of the flat surfaces has a printed slow wave circuit in the form of at least one tightly wound equiangular spiral arm between the electron gun and the collector. Though a high power amplifier needs focusing means, a small amplifier made according to the invention for operation at low power needs no electron beam focusing between electron gun and collector. The invention can be combined with a phased array to serve as an amplifier in place on the array; one such amplifier is provided for each radiator element. The invention can serve as amplifier and radiator by designing the spiral arm for radiation.

nu smears' [451 ,any i7, i973 Mnited @taies Patent t191 jasper, ,lin etal,

..315/3.6 3/1954 Tiley.....................................315/5 [541rnAvELrNc WAVE ruimt wim PLANAR 3,571,651 EQUIANGULAR srinni, Stow WAVE2672572 cmcuir [75] Inventors: Louis J. Jasper, Jr., Neptune City;

Primary Examiner--Rudolph V. Rolinec Char. M D S n N t AssistantExaminer-Saxfield Chatmomjr.

es e m Si eP une; Attorne -Harr M. S' `t Ed d ll Frederick 1B.Sherburne, Oceanport, et al y y dragovl Z War J Ke y al1 of NJ.

[73] Assignee: The United States oli America as [57] ABSTRACT Thisinvention concerns a more compact, lower-cost, lower-voltage,broader-band, traveling wave amplifier represented lby the Secretary otthe Army, Washington, D.C. that has a cylindrical electron gun betweenclosely [.22] Filed' May l5 w72 spaced, parallel, flat surfaces of apair of ceramic disks [2l] Appl. No.: 253,043 and a ring collectorjoined to the disks concentric with the electron gun. At least one ofthe flat surfaces has a printed slow wave circuit in the form of atleast one tightly wound equiangular spiral arm between the electron gunand the collector. Though a high power amplifier needs focusing means, asmall amplifier made according to the invention for operation at lowpower needs no electron beam focusing between electron gun [56]References Cited UNITED STATES PATENTS and collector. The invention canbe combined with a phased array to serve as an amplier in place on thear- 3,305,752 2/1967 315/5 X 3,258,702 6/l966 3,153,742 l0/l9642,617,961 ll/l952 ray; one such amplifier is provided for each radiatorel Hart..........

ement, The invention can serve as amplifier and radia tor by designingthe spiral arm for radiation.

Kluver 4 Claims, 4l Drawing Figures Patnted July 17, 1973 I 3,746,915

2 Sheets-Sheetl Patented yJuly 17, 1973 3,746,915

-2 Sheets-Sheet 2 f F/G; 3

FIG. 4

BACKGROUND OF THE INVENTION In any traveling wave amplifier device, abeam of electrons gives up energy to a high frequency signal to amplifythe Sianalfhesianalis nlgpaaatsiillgnaaitcuitous path at essentiallyfree space velocity, but the advance of the circuitous path in adirection normal to the path is a small fraction ofthe length ofthecircuitous path. The signal that is propagated along the circuitous pathis accompanied by a wave propagated normal to the path with a slow phasevelocity that is a fraction of the free space velocity of the signalalong the circuitous path. The beam of electrons is propelled normal tothe circuitous path at essentially the phase velocity of the slow waveand along intense electric field regions of the slow wave. The electronbeam is accelerated by the electron gun to the slow-wave phase velocitybefore introduction into the interaction region and then traverses theinteraction region. The interaction region is equipotential. Magnetic orelectrostatic focusing is applied to the electron beam along theinteraction region. During interaction between the beam electrons andthe propagated slow wave, interacting electrons are velocity-modulatedcausing bunching of electrons along the transiting beam and a generalslowing of the beam. When operating properly, there is transfer ofenergy from the electron beam to the signal, amplifying the signal. Thegain is related to the length of the interaction region in wavelengthsand the beam current density that interacts with the propagated wave.

A common form of slow-wave guiding means for a traveling wave tube is awire helix that conducts the high frequency signal energy andconcomitantly propagates a slow electromagnetic wave produced by thesignal energy along the helix at a phase velocity substantially matchedby the velocity of the electron beam. The wire helix surrounds theelectron beam path. In another type of traveling wave tube, a structurewhich guides the wave crosses back and forth across the path of anelectron beam, but provides an unobstructed path for the electron beamalong electric field regions of the propagated signal wave. As thesignal propagates along the helix or waveguiding structure, the electronbeam, in synchronism with the longitudinal phase velocity of the signal,interacts with the electric field ofthe propagated slow wave in suchmanner that the electron beam becomes velocity-modulated and bunched andthe signal grows in strength.

Known types of traveling wave tube amplifier devices have one or moredisadvantages which include high cost, complex design, comparativelynarrow band, operate at high voltage and are larger and heavier thandesirable. Improvement of any one or more of these characteristics isgenerally desirable. More particularly, there is need for lighterweight, broadband, lower voltage, efficient, economical expendablebatteryoperated low-power units for expendable jammers or otherexpendable high frequency electronic equipments. Also, there is needlfor improved modulator and amplifier devices that can be used in placein phased array antenna assemblies. 'Also there is need for a devicethat can be used as amplifier, modulator and radia- (01.

SUMMARY OF THE INVENTION An object of this invention is to provide amore compact traveling wave tube amplifier device.

A further object is to provide a lighter weight, lowpower traveling wavetube amplifier device.

A further object is to provide a low voltage traveling wave tubeamplifier device.

A further object is to provide a traveling wave tube device with a highperveance electron gun.

A further object is to provide a printed circuit traveling wave tubeamplifier device.

A further object is to provide a superior, high-power traveling wavetube amplifier device.

A further object is to provide a more economical and more efficienttraveling wave tube amplifier device.

A further object is to provide a generally superior broadband travelingwave tube amplifier device.

A further object is to provide a traveling wave tube device for use inplace in a phased array antenna assembly as a superior amplifier.

A further object is to provide a traveling wave tu be for use in anantenna assembly as combination amplifier and modulator, or as amplifierand antenna element.

A traveling wave tube amplifier device according to this inventionincludes a pair of dielectric disks and a ring member sealed to theperimeters of both disks. The disks are spaced apart a fraction of aninch. A cy` lindrical electron gun is supported between the disks andwith its axis normal to the flat parallel faces of the respective disks.The ring member serves as electron beam collector. Slow-wave propagationcircuit means including at least one flat equiangular spiral armconductor is printed on the inner surface of at least one of the disksbetween the electron gun and the collector. Coaxial connections areprovided to the inner and outer ends of the spiral conductor. If beamfocusing is needed in the interaction region, it is provided in the formof periodic permanent magnet (PPM) means against the outer sides of thedisks or by use of the Einzel lens effect, i.e. electrostatic focusingusing the spiral conductors as lens means.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a plan view of thedevice shown in FIG. 2;

FIG. 2 is a section of a short circular traveling wave tube according tothis invention taken along line 2-2 of FIG. l;

FIG. 3 is a section taken on line 3-3 of FIG. 2 showing a flatequiangular spiral conductor slow wave cir cuit and anode; and

FIG. 4 shows an interlaced double spiral for use in this invention.

The embodiment shown in FIGS. 1 and 2 is a short cylindrical travelingwave tube device 8 that includes a pair of parallel dielectric diskplates l0, 1l of alumina ceramic or other suitable dielectric material,spaced apart as little as inch in a low power tube, and a conductor ringl2 joined to both plates and together confining a short cylindricalvacuum chamber 13. A cylindrical electron gun 14 is supported betweenthe plates for propelling a high perveance sheet beam radially, overfully 360 around the electron gun to the collector ring. Amicroperveance of 50 is reasonable because this arrangement lends itselfto high-current lowvoltage operation. A flat equiangular spiral armconductor slow-wave circuit l5, as shown in FIG. 3, is printed on theplate to provide a slow radiallypropagated wave with a substantiallycircular wavefront when a high frequency signal is coax-coupled into theinner end of the spiral and is coax-coupled out of the other end of thespiral. The equiangular spiral conductor increases in width from theinner end to the outer end. Under the proper operating conditions, theradial electron beam and the radially propagated traveling wave are insynchronism and interact; the signal energy conducted through the spiralextracts energy from the beam and is amplified while transiting thespiral. The plate heat-sinks the printed spiral conductor.

Electron gun 14 includes a cathode 16, a heater 17 within the cathode,an electrode 18 in the form of a cylindrical wire grid surrounding thecathode, and accelerating electrodes 19, 20 printed on the two platesand surrounding the electrode 18. Cathode 16 is in the form of a shortcircular cylinder 16a of emitting material with non-emitting conductivecircular flanges 16b and 16C at its ends. The flanges 16b and 16C, atthe same potential as the emitting cylinder 16a, shield against cathodeemission from the emitting cylinder ends directly toward the plates 10,11. The diameter and length of the cathode is designed so that thecathode surface is large enough to supply the required beam current.Accelerating anodes 19 and 20 are identical flat conductive ringsprinted on the plates 10, 11. Wire grid electrode 18 is anonintercepting grid or it is formed of very thin wire and hascomparatively large spaces to offer minimal obstruction to the electronbeam. The cathode, wire grid, and accelerating anodes are coaxial withone another and with the ring 12. Direct current power supply means forand connections to cathode 16, wire grid 18, accelerating anodes 19, 20and ring 12 are omitted from the drawing. Similarly AC or DC powersupply means for the heater 17 are omitted. The collector ring 12 ischannel shaped in cross section. One advantage of the channel-shapedcollector is that it captures more of the beam electrons that mightstrike the plates 10, 11 since after the beam passes through thesubstantially equipotential field in the interaction region, beam spreadincreases. Another advantage of the channel shape is that the collector12 can be heat-sinked by the plates 10, 1l.

The choice of electrodes and the number of electrodes in the electrongun and their design are not part ofthis invention. The electron gunrequires an accelerating anode. A control electrode, a beam shapingelectrode and an additional accelerating anode may be included in theelectron gun design. In the disclosed embodiment, the electrode I8 canbe used either to switch off` the beam for pulse operation or tootherwise modulate the beam. Alternatively, the electrode 18 may be usedas the accelerating anode. The printed anodes 19, 20 may be replaced bya second wire grid electrode or electrode I8 may be replaced by anotherpair of printed annular electrodes coaxial with accelerating anodes 19,20. In operation, the pair of electrodes 19, 20 are connected in commonand are at the same potential. Any combination of the well known designoptions for electron guns may be adapted to this invention.

Though the structure shown in FIG. l has the electron gun at the centerand the collector at the perimeter. this invention also contemplates astructure where the electron gun forms the perimeter and the collectoris at the center, for specialized applications.

The traveling wave tube device in FIG. 1 has a slow wave circuit on onlyone of the plates; the opposed plate is provided with a printed flatannulus 22 of conductive material having the same radial dimensions asthe slow wave circuit l5. In operation, the slow wave circuit 15 andopposed conductor annulus 22 are at the same DC potential relative tothe cathode. Alternatively, if there is slow wave circuitry on each ofthe plates l0, l1, the slow wave circuitry on both plates are identical,and are maintained at the same DC potential during operation. Byapplying power DC potentials to the cathode, wire grid, acceleratinganodes, the slow wave circuitry and the collector, a high perveancesheet beam is obtained within the radial span of the slow wavecircuitry. For low power and/or high frequency operation, no magneticnor electrostatic focusing is needed, provided the radius of the ring l2is limited to a few inches. For high power and/or low frequencyoperation, electrostatic or magnetic focusing is required. The spacingbetween the plates is small in order that a large percentage of theelectron beam current interact with the slow wave(s) propagated by thespiral(s); interaction occurs close to the slow wave circuit(s). Thisspacing is as small as possible consistent with the length of thecathode.

The slow wave propagating structure 15, shown in FIG. 3, is a printedequiangular spiral conductor of constantly increasing width betweeninner and outer ends. Since the amplification process increases thepower of the signal as it progresses along the spiral from the inner endto the outer end, the increasing width accommodates the increasingsignal power. The equation describing the outer edge ofthe spiral arm ispl P06005 and the equation describing the inner edge of the spiral armis where e represents the base of natural logarithms, p and rb are polarcoordinates and -ais a constant and is a fraction on the order of0.01,-p0 and 0 are the initial radial and phase coordinates. Theequations are related as follows The constant k is a measure of theangular width of the spiral arm. The constant -adetermines the tightnessof the spiral; a lower value for the constant -aproduces a tighterspiral. If the spiral is tight, the slow wave propagated by the spiralhas a circular wave front. The wave front becomes elliptical if -aisincreased sufficiently; phase velocity ofthe propagated wave is directlyproportional to -a. The radial electromagnetic wave propagated by thespiral circuit is very slow with a phase vclocity on the order of l/5Othe velocity of light. For such slow phase velocity, acceleratingvoltage can be low, i.e. under 500 volts. Also slow phase velocity meanssmall circuit wavelength. Since gain is proportional to the number ofcircuit wavelengths, a high gain device can be fabricated in a small orcompact package.

The length L of the spiral, defined as the distance from origin to theoutermost length of the spiral arm,

as measured along the center of the spiral arm can be expressedapproximately as L= 1li/112+ 1)(p-Po) The characteristics of the spiralstructure are specified by the three variables; spiral arm length,constant p0 and constant k.

To realize frequency-independent performance, the length of the spiralarm must be at least one wavelength at the lowest frequency; forinteraction with a radial electron beam, as described, the length of thespiral arm should be many circuit wavelengths. As a practical matter,the number of circuit wavelengths would be on the order of 20.

ln FIG. 4 there is shown an alternative slow wave propagation structureThe structure includes identical interlaced spiral arms 15a and 15bhaving equal values of p0 at their origins but with their originsangularly separated 180 relative to the axis. Their inner ends areconductively joined by printed bridging conductor 15C. Either one of thespirals can be described by the relationships given above. The outer andinner edges p3 and p4 of the other of the two spiral arms are definedasl p3 :pueda-n) P4 Pii"(b" d) kpn The outer ends of the spiral armsterminate 180 apart. However, it is feasible to extend one of the spiralarms 180 in order to use one output connection to the other ends of bothspiral arms. Comparing two slow wave circuits of the same inside andoutside radii, one slow wave circuit having one spiral arm and the otherslow wave circuit having two spiral arms and both slow wave circuitshaving the same' total number of turns, the value -afor the doublespiral circuit is twice that for the single spiral circuit and the phasevelocity of the wave propagated by the double spiral circuit is twicethat propagated by the single spiral circuit which requires anaccelerating voltage which is greater by a factor of 4.

A successful method that was used to print the conductor spirals andannular electrodes shown in FIG. 3

on dielectric substrate included the following steps.

The spiral curves p1, p2 etc. were plotted on drawing paper on a largerscale than required for the structure. A computer was used in theplotting of the spirals. Circles were drawn for the annularelectrode(s). Then for each spiral arm and for the annular electrodes,the space betweennthe inner and outer edges were blacked in and thedrawing was photographed to provide a transparency with the imagereduced to the proper size. Each dielectric substrate had a layer ofconductor deposited on one surface by one of the known prior arttechniques that include sputtering, vacuum deposition, plasmadeposition, etc. Then using the transparency, the conductor-overlaiddielectric substrate was photoetched. The printed circuit aspect of theinvention contributes reproducibility and economy. Electricalconnections, not shown, arevbrought out through holes formed in theceramic and subsequently sealed.

An equiangular spiral arm has advantages over other kinds of spirals inthis invention. ln the equiangular spiral, phase velocity is independentof radial distance;

where v, is phase velocity c is the velocity of light -adetermines thetightness of the spiral as stated previ- Phase velocity dependent on ris undesirable.

A significant property of the equiangular spiral is its log-periodicity.The ratio ofthe arm widths for any two consecutive turns is a constant.Because of the logperiodic property, very wide frequency (multi-octave)bandwidths are attainable. The spiral circuit is essentially frequencyindependent over a wide frequency bandwidth and exhibits frequencyindependent impedance characteristics.

Where the equiangular spiral as described is designed for considerablegain and is many wavelengths long, electrons that interact with a slowwave propagated by the spiral may give up enough energy to slowsufficiently to fall out of synchronism. Phase taper can be designedinto the spiral by decreasing the value -awith increasing radius justenough to maintain synchronism between the interacting electron beam andthe propagated slow wave.

The described embodiment of the invention may be made with one spiralarm on one of the plates 10, 11 or with two or more spiral arms on theone plate. If there are two or more spiral arms, their inner ends areangularly separated 2 1r divided by the number of spiral arms.Alternatively each of the two plates may have one spiral arm or an equalnumber of spiral arms. All of the spiral arms are essentially identical.As stated previously, if the spiral arms are many wavelengths long, thespirals may be terminated contiguously and joined together, using phasematching techniques well known in the art, so that only one outputconnection is used for the outer ends of the plurality of spiral arms.During operation a small percentage of the beam electrons may strike thetwo plates. Those electrons that strike the plates return through thespiral conductors and no significant electric charge accumulates on theceramic.

A characteristic of the radial electron beam is that the current densitydecreases with increasing radial distance. This characteristicsimplifies the focusing problem. The beam spreads as it transits outwardreducing the perveance per unit distance or unit perveance. Thespreading of the beam counteracts debunching of the beam as the beamgives up energy to the wave. Debunching is undesirable because it isaccompanied by degradation in gain and thus a reduction in signalstrength.

Bandwidth on the order of several octaves is obtainable with thisinvention. Outside radius of the slow wave circuit determines upperfrequency cutoff', inside diameter and maximum width ol' the spiral armdeter mines the lower frequency cutoff. Circuit radii' ol'iiboiii 1.5,3, and 4.5 inches have upper frequency cutoffs nl approximately 2000,1000, and 50() megahertz, respectively. Bandwidths are about 1000-2000,500-1000, and l00500 megahertz, respectively.

The spiral arms have antenna properties and will radiate through theceramic when the circumference of the spiral circuit approaches 2A.,where )t0 is free space wavelength. ln order to extend the higherfrequency cutoff range and block radiation from the device, coppershield disks 30, 32 may be secured to the outer sur` faces of theceramic disks l0, 1l. The copper shields are needed only where radiationwould be a problem. However, the device can be made to operate with bothamplifier and radiation properties by designing the spiral arm(s) forenhanced radiation and by omitting one or two of the two copper shields.lri this form, the clevice can be used as combined amplifier andradiator element on an antenna array, replacing the conventionalradiator element.

l lf beam focusing is required to resist beam spreading because of highbeam current level or because outside diameter of the slow wave circuitis large, periodic permanent magnet (PPM) means or Einzel lenselectrostatic focusing may be used. A PPM, not shown, can take the fromform a washer-like flat annulus of about the same inside and outsideradii as the slow wave circuit and have a set of concentric ring zonesof successively opposite polarity in the axial direction. It can beformed in a ceramic matrix using well known prior art techniques. EachPPM would be supported coaxally with the spiral structure against theouter faces of the respective plates 10 and 11 or copper shields 30, 32where such shields are used. DC and RF connections can be brought outthrough holes drilled in the PPM if the connections cannot be broughtout conveniently from within or outside of the annular PPM. lf Einzellens electrostatic focusing is used, it requires a plurality of spiralarms on each plate and also requires that the spiral arms not beconductively joined at their inner nor outer ends. Assuming two spiralarms on each plate, each of the two spirals of each plate are atselected upper and lower DC voltages that are equally displaced from theselected voltage of the slow wave circuit. For example, if the meanvoltage of the slow wave circuit is to be 200 volts positive to thecathode, one of the spirals of each plate would be at 175 volts and theother of the spirals of each plate would be at 225 volts therebyproviding the desired mean voltage of 200 volts. Matching impedancemeans is needed when using this focusing arrangement; matching impedancemeans may be printed on each of the plates 10, l1.

Based upon Traveling Wave Tubes" by Pierce published by VanNostrand1950, pages 16 and 252-255, the theoretical gain from a traveling wavetube device is approximately G=A+BCN w 21r X frequency v, Vn/(500) X C=phase velocity of circuit wave c velocity of light V0 voltage ofelectron beam I., beam current P RF power The impedance of the electronbeam is equal to V/1l The interaction impedance K Er2/2P The dominantmode ofinteraction that occurs in this invention is characterized bycircular wave fronts. Only the dominant mode grows in amplitude since itis the only mode with circular wavefronts. Hence, there is no problemwith higher harmonics in this invention.

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and describedl for obviousmodifications will occur to a person skilled in the art.

What is claimed is:

1. A radial interaction traveling wave tube for a predetermined bandcomprising:

a circular collector electrode and two flat members joined together toform a sealed space;

means supported by the flat members within the sealed space coaxial withthe collector electrode for radiating a 360 degree electron beam towardthe collector electrode; spiral conductor means printed on the insidesurface of one of said flat members for propagating an electromagneticwave, coaxial with the collector electrode and said means propagatingmicrowave energy producing a circular wavefront, the radial width of thespiral along its length being greatest at the outer end and being ofconstantly increasing width from its inner and to its outer end, one ofthe edges of the spiral describing a curve according to the relationshipp1=p0ea and the other of the edges of the spiral describing a curveaccording to VtheYY relationship p2=p0ea il where e is the base ofnatural logarithms and p0, p1, p2, QS and Q50 are polar coordinates, andp0 and da@ are coordinates of the inner end of the spiral; a being aconstant on the order of 0.01 and manifesting a tighter or looser spiralas the magnitude selected for a is decreased or increased respectively;the length of said spiral being a plurality of circuit wavelengths atthe lowest frequency signal of said predetermined band, whereby saidconductor has the configuration of an equiangular spiral and propagatescircular wavefronts for signal frequencies within the predeterminedband.

2. A traveling wave tube as defined in claim l further including asecond equiangular spiral conductor interlaced with said first-mentionedequiangular spiral conductor and wherein said spiral conductors areconductively connected at their inner ends.

3. A radial interaction traveling wave tube as defined in claim lfurther comprising a second spiral conductor, identical to and coaxialwith thc first-recited spiral conductor, printed on the inside surfaceof the other member.

4. A radial interaction traveling wave tube as defined in claim 3wherein said flat members are ceramic and further including a coppershield secured to the outer side of at least one of the flat members.

u l lr :if

1. A radial interaction traveling wave tube for a predetermined bandcomprising: a circular collector electrode and two flat members joinedtogether to form a sealed space; means supported by the flat memberswithin the sealed space coaxial with the collector electrode forradiating a 360 degree electron beam toward the collector electrode; aspiral conductor means printed on the inside surface of one of said flatmembers for propagating an electromagnetic wave, coaxial with thecollector electrode and said means propagating microwave energyproducing a circular wavefront, the radial width of the spiral along itslength being greatest at the outer end and being of constantlyincreasing width from its inner and to its outer end, one of the edgesof the spiral describing a curve according to the relationship Rho 1 Rho0e a and the other of the edges of the spiral describing a curveaccording to the relationship Rho 2 Rho 0e a( ) where e is the base ofnatural logarithms and Rho 0, Rho 1, Rho 2, phi and 0 are polarcoordinates, and Rho 0 and 0 are coordinates of the inner end of thespiral; ''''a'''' being a constant on the order of 0.01 and manifestinga tighter or looser spiral as the magnitude selected for ''''a'''' isdecreased or increased respectively; the length of said spiral being aplurality of circuit wavelengths at the lowest frequency signal of saidpredetermined band, whereby said conductor has the configuration of anequiangular spiral and propagates circular wavefronts for signalfrequencies within the predetermined band.
 2. A traveling wave tube asdefined in claim 1 further including a second equiangular spiralconductor interlaced with said first-mentioned equiangular spiralconductor and wherein said spiral conductors are conductively connectedat their inner ends.
 3. A radial interaction traveling wave tube asdefined in claim 1 further comprising a second spiral conductor,identical to and coaxial with the first-Recited spiral conductor,printed on the inside surface of the other member.
 4. A radialinteraction traveling wave tube as defined in claim 3 wherein said flatmembers are ceramic and further including a copper shield secured to theouter side of at least one of the flat members.